?stage=live not working

All of a sudden my working site (www.tecca.com.au) started throwing 500 internal server error. At least I am able to login now in the admin panel and see the draft site (?stage=stage) however the published site(?stage=live) is still showing 500 internal server error. Any clues?

Is is better to modify in main.php or _config.php? Can you also be more specific about which _config.php to modify as there are so many in different directories? The server error message says something about :